Various types of positioning apparatus are used in which commutator-type motors, typically permanent magnet d-c motors, are used for moving a positioned element in one or two limit positions, and in which the positioning motor is manually controlled to ON, or moving position, while stopping automatically when the limit position is being reached, without, however, being turned OFF by a limit switch. Circuit arrangements of this type are used, for example, for projection and retraction of telescopic antennae on vehicles, or in other automotive applications, for example for automatic window closures, operation of sliding windows, or the like; or for any other use in which a positioned, movable element can move between two predetermined end or limit positions.
One circuit arrangement for a motor-driven antenna uses a relay to connect the antenna extension motor, and at the same time energizes electronic timing circuits. The ON switch of the motor can be coupled to an automotive radio which, upon turning the radio ON, simultaneously causes the antenna to be projected; upon turning the radio OFF, the antenna is retracted. It is difficult to control projection or retraction of a motor based on time alone, since, due to variations in supply voltage of the on-board network of a vehicle, changes in temperature or other interference, the timing elements must be so designed that the motor remains energized for a period of time which is longer than that for operation under optimum conditions. The motor, or any associated drive gearing coupled to the antenna, for example, should not be overloaded. To prevent such overloading, it has been proposed to introduce a slip clutch between the motor and the antenna. This is a comparatively expensive solution. Additionally, due to accumulation of dirt, icing of the antenna, or the like, projection or retraction can be delayed to such an extent that even the additional safety or reserve time is not sufficient to completely project or retract the antenna; specifically, frequently an antenna which had been subject to icing will not retract completely but will remain in partially projected position. If the reserve or safety factor time is dimensioned sufficiently long so that the antenna will reliably retract under even the most severe icing or contamination conditions, the time of operation of the motor beyond that for retraction of the antenna when it is not iced, and clean, will be excessively long. The noise level may be annoying, and the mechanical system is excessively loaded.
It has been proposed to turn OFF an antenna drive by providing a limit switch, so that the motor is turned OFF only when a certain limit position has been reached. If, however, the antenna, for example due to icing or excessive accumulation of road dirt and the like, cannot be completely projected or retracted, overloading of the drive motor may result, with possible damage or destruction thereto. Overload protection can be provided, of course, including thermal overload protection. This, however, is comparatively expensive and also subject to malfunction under widely varying ambient temperatures. Additionally, the switches which are necessary are precision switches which are expensive to manufacture and install. They are not well suited for an automotive environment, since they are subject to damage by corrosion, wide temperature variations and the like, and must be designed for high-current carrying capacity at the low voltage levels pertaining in automotive on-board vehicular networks. Reliable operation for a long period of time thus cannot be insured.